The present invention relates to a digital apparatus for controlling members by eddy currents and it can be used in non-destructive control processes.
It is known that the control of a metal member by eddy currents involves the study of the variations of the currents induced in said member by the magnetic field of a primary winding traversed by an alternating exciting current. On their return, these induced currents produce a field which opposes the induction field and which acts on the impedance of a secondary winding. The assembly of the primary and secondary windings forms the sensor in which or in front of which is displaced the member to be controlled. Any defect or fault in the latter occurring at the level of the sensor (change of dimension, variation in the electrical conductivity, variation in the magnetic permeability, cracks, etc) modifies the phase and intensity of the eddy currents and correlatively changes the impedance of the secondary winding.
For sensitivity reasons, the receiving coil is constituted by two secondary windings connected in opposition in such a way that the measuring signal is the asymmetrical voltage of the two windings. The measurement is differential if the two windings examine two adjacent zones of the member to be controlled and is absolute if only one of the two windings is in the presence of said member (the secondary winding then has a reference standard member). The voltage supplied by the sensor is amplified and then analysed in its resistive (or true) component X and in its reactive (or imaginary) component Y. The alternating voltage supplied by the sensor is then represented on the screen of a cathode tube by a point of cartesian coordinates X and Y.
Due to the slight asymmetries between the two parts of the secondary winding, even when there is no fault in the member to be controlled, the sensor supplies a residual voltage which may falsify the measurement. Thus, an attempt is made to eliminate this residual voltage by means of an auxiliary correcting or compensating circuit comprising on the one hand a compensating voltage generator, whose frequency is equal to that of the exciting voltage and whose amplitude and phase are regulatable and on the other hand a differential amplifier with two inputs, one receiving the compensating voltage and the other the voltage supplied by the sensor. The regulation of the amplitude and phase of the compensating voltage is carried out in such a way that in the presence of a member which is considered to be satisfactory, the amplifier supplies a zero voltage, at least at the exciting frequency. It is this corrected voltage which is finally analysed. To this end, a circuit is used, which generally comprises two periodic samplers both of which operate at the frequency of the exciting voltage, but in quadrature with respect to one another. These samplers receive a control pulse from the exciting voltage generator, said pulse passing along a delay line, thus permitting the regulation of the sampling times.
Finally, it is standard practice in such apparatuses to provide a means for rotating the measurement points displayed on the cathode tube screen. In general, this means consists of circuits able to calculate quantities of the form X cos .theta.-Y sin .theta. and X sin .theta.+Y cos .theta., .theta. being the desired rotation angle.
In connection with said special means and apparatus, reference can be made to U.S. Pat. No. 3,229,198 of H. L. LIBBY, granted on Jan. 11, 1966 and entitled "Eddy current nondestructive testing device for measuring multiple parameter variables of a metal sample".